A multiphase converter has a plurality of parallel connected conversion units. The number of conversion units that operate changes according to predetermined conditions in order to maximize efficiency. Patent Document 1 describes a power supply system that includes a plurality of power supplies connected in parallel and an operating number controller that controls the number of the power supplies in operation. The operating number controller determines the number of the power supplies operating based on efficiency relative to the load current. The technique described in Patent Document 1 is also described in Non-Patent Document 1. The technique described in Patent Document 3 is also described in Patent Documents 2 and 8. The technique described in Patent Document 4 is also described in Patent Documents 5 and 6.